Method for purification of ethanol



METHUD FR PURlFlCATlN F ETHANOL Howard Greltel and Clifton El. Goddin, Jr., Tulsa, Okla., assignors to Pan American Petroleum Corporation, a corporation of Delaware Original application December Z7, 1952, Serial No. 328,1tl. Divided and this application .lanuary 14, 1957, Serial No. 634,123

5 Claims. (Cl. 2mn-40) Our invention relates to a novel method for the separation of various oxygenated organic compounds from one another. More particularly, it pertains to a process for recovering ethanol in highly purified form from aqueous mixtures containing methanol.

While mixtures of the type contemplated by our invention may be found to exist as product streams from other industrial operations, the process of our invention is directed principally to certain fractions of the primary water-soluble chemicals -stream produced in hydrocarbon synthesis involving the reduction of carbon monoxide with hydrogen in the presence of a iluidized iron catalyst. In the hydrocarbon synthesis plants now designed for commercial purposes, having capacities of the order of about 6,000 barrels of liquid hydrocarbons per day, the nonacid chemicals present in the aqueous mixtures produced constitute about 75 percent of the total water-soluble chemicals formed, or about 320,000 pounds per day. Accordingly, for economical and other reasons, it is essential that those chemicals be recovered each in as highly purified form as possible.

A mixture typical of those with which we are con cerned and the relative concentration in which each component thereof is present are indicated below:

ln recovering a mixture such as that specifically described immediately above, the primary water produced in hydrocarbon synthesis was first subjected to a fractional distillation step (l) to separate the oxygenated chemicals in two groups, the alcohols, aldehydes, and ketones being taken off overhead, and the acids being removed in aqueous solution from the bottom of the still. The overhead thus obtained generally contains from about l5 to 2O weight percent water. This fraction was next distilled (2) to take overhead the l-propanol and lower boiling compounds, and l-butanol and heavier compounds were withdrawn as a bottom stream. After removing acetaldehyde from the last-mentioned overhead, a mixture having the typical composition given in Table I was obtained. The latter was next subjected to a fractionation step (3) designed to take overhead as distillate methanol and lighter materials, such as acetone, propionaldehyde, etc., leaving the butyraldehyde, ethanol,

2,826,536 Patented Mar. 11, 1958 and heavier fraction as bottoms. Because of the relatively high proportion of acetone to methanol in the feed to the aforesaid distillation step (3), it was expected that all of the methanol would be removed with the acetone in the form of an azeotrope boiling at 55.7 C. With the type of mixtures involved, however, it was found in actual practice that the removal of methanol in the expected fashion was impossible to accomplish in columns designed for commercial operation. This we have found to be true regardless of the fact that acetone is present in a concentration of from about ten to twenty times greater than that of the methanol, notwithstanding the additional fact that an acetone-methanol ratio of only 7.3:1 is required to satisfy the acetone-methanol azeotrope. Thus, while the key azeotropic systems involved in such a separation, i. e., acetone-methanol (B. P. 55.7 C.) and ethanol-butyraldehyde-water (B. P. 67.2 C.), boil approximately ten degrees apart, it was found that with mixtures of the type discussed above, appreciable quantities of methanol passed into the bottoms along with the butyraldchyde, ethanol, and higher boiling products.

The last-mentioned bottoms fraction referred to above was then subjected to a further distillation operation (4) in which the ethanol and lighter components were taken overhead and the l-propanol and heavier compounds were removed as bottoms. The expression ethanol and lighter components as used herein is intended to refer to components which form lower boiling azeotropes with ethanol and/or water. The overhead containing ethanol was next subjected to a fractional distillation step (5) in which distillation was effected in the presence of a high concentration of water in the distillation zone. Ethanol under such conditions passed into the bottoms, and the latter thereafter again subjected to fractionation (6) to obtain product ethanol. Ethanol produced in this manner, however, contained methanol in concentrations as high as about 1 weight percent, Whereas specilications for high grade ethanol permit not more than about .04 weight percent of methanol.

Although ordinarily it is a relatively simple task to separate methanol from ethanol, we have found with mixtures of the type contemplated that such a step is extremely dillicult to accomplish, while at the same time causing the bulk of the compounds such as butyraldehyde, methyl ethyl ketone, ethyl acetate, and similar materials to remain in the bottoms with the ethanol. Accordingly, it is to this object, i. e., removing methanol overhead while retaining butyraldehyde and/ or the aforesaid ketone, ester, and similar compounds in the bottoms with the ethanol, to which process our present invention is primarily directed.

We have now discovered that the cause of the methanol passing into the bottoms under the condition set forth above is due to the presence of relatively large concentrations of water in the nonacid feed to distillation step (3) mentioned above, i. e., the operation designed to separate methanol and light components from butyraldehyde, ethanol, and heavier compounds. This diticulty we have found can be overcome in effect by reversing the usual sequence of distillation steps (3) and (4) discusscd above in connection with the prior art methods that have been used in separating mixtures of this type. ln recovering ethanol substantially free of methanol from such mixtures, while retaining butyraldehyde, methyl ethyl ketone, ethyl acetate and similar compounds in the bottoms with ethanol, we have found that the water content of the latter should be reduced to a value of not more than about 7 weight percent. Also, it should be pointed out that at such water concentrations, the total low molecular weight alcohols, i. e., methanol, ethanol, and isopropyl alcohol concentrations, should not be 'greater than about "60 Weight percent (dry basis) with lmethanol being in aconcentration not in excess of from about l to about percent based on the weight of the ethanol. ln this connection, the expression low molecular weight alcohols as used herein is intended to lreferto methanol and ethanol and mayinclude isopropyl alcohol. The acetone content of the mixture may vary -from 'about l0 to about 25 percent (dry basis), and the total concentration of water-soluble keton'es present may range'fro'm about 20 to about 50 percent (dry basis) with acetone generally amountingto from about 50 to about `75 percent of 'thetotal ketones.

In accordance with a preferred embodiment of 'our invention, a feed having a composition corresponding to Vthatused in 'the aforesaid distillation step (3) is subjected to fractionation under conditions such that ethanol and lighter compounds pass 'overhead with the methanol,

While lprop'anol, as'ubstantial proportion of the water,

'suitable for the removal overhead ofthe methanol and 'lighter'fraction, 'and the withdrawal of the butyraldehyde,

ethanol, and heavier fraction from the bottom of the ldistillation column. This bottoms fraction may then be processed in accordance with distillation steps (5) and (6) discussed above to secure a highly purified product ethanol stream, i. e., containing less than about 0.05 weight lpercent methanol.

In order to further illustrate the process of our invention, reference is made to the accompanying drawing wherein a`feed having the composition specically 'set forth in Tablel is added to column 2 through line 4. Under the conditions of operation, 1-propano1 and high-'boiling impurities together with the bulk of the water are withdrawn as bottoms from the system through line 6. Vapors are withdrawn overhead from the column at a temperature of about 73 C. through line 8, condenserlll, and a portion of the resulting condensate returned to the column as reflux through line 12. The bulk of the distillate, which amounts to about 73 weight percent `of the feed, `is sent through line 14 to column 16. vThe composition of the feed to column 16 is as Column 16 is 'operated at a-top'tower temperature of 'about 54 C. with vvapors of methanol, acetone, and

propionaldehydepassing overhead'through line 18 and condenser 20, a 'portion of the condensate being returned to'the column through line 22 as reux. The remainder of thecondensate is sent to further refining in a suitable ymethanol-acetone puricationsystem through line 24.

Bottoms at a temperature of about 78 C., containing ethanol and higher boiling compounds, is withdrawn `through line 26 and sent to an intermediate section of column 28 Where it is subjected to vfractional distillation in the presence of from about 90 to 99 mol percent water. lThis concentration in the distillation zone is maintained by adding water through line 30. vThe amount of net distillate drawol is carefully controlled -so that ethanol of the desired purity is produced. The

quantity of heat supplied to the column is normally in the range of 100 to 150 B. t. u. per pound of bottoms, A minimum amount of heat `is required to secure satisfactory stripping of trace impurities from the ethanol at this point. The use of excessive quantities of heat impairs the efficiency of the column due primarily to decreasing water concentrations in the column. The ternperatures employed may vary, but for the majority of purposes we have found that the bottom tower temperatures range from about to about 98 C., preferably 96 C. (corrected to atmospheric pressure), and the overhead temperatures range from about 73 to about 78 C., preferably about 76 C. Under these conditions, ester and carbonyl impurities, together with isopropyl alcohol and some ethanol, are carried overhead through line 32, condenser 34, and this distillate sent to further refining throu'ghline 36. A portion of this stream is returned as reflux to the top of the column through linef.

Dilute ethanol withdrawn from the base of column 28 through line 39 is sent to a conventional fractionating column 40 wherein a constant boiling mixture of ethanol 'and water is removed in the form of a side stream through line 'd'2 at atemperature of about 78 C. The product ethanol thus obtained contains not more than about 0.04 weight percent of methanol. The overhead, which is largely ethanol containing low-boiling impurities, is withdrawn through line 44 and condenser 46. A portion of thisstream is retluxed to the column through line 48 'and the remainder, containing from 50 to 75 percent of 'the methanol fed to the column, is recycled to column 16 through line Sti. Purity of the ethanol side draw through line 42 may be controlled by the quantity of liquid taken overhead through line 50. Bottoms from column 40, consisting essentially of water, may be re- 'cycled to column 28 through line 52 for use as dilution water in the extractive distillation step. Build up of 'objectionable concentrations of heavy impurities in the system may be prevented by discharging a portion of the bottoms through line 54.

Using a feed of essentially the same composition as that employed in the operation of column 2 and running the latter so as to take overhead only methanol and lower boiling compounds in accordance with prior art procedures, the methanol content 0f the product ethanol ultimately obtained is found to be about 1 weight percent. In this regard, one of the outstanding advantages of our process over the prior art is the fact that by practicing our invention, it is possible to `secure substantially methanol-free ethanol, from mixtures of the type herein contemplated, by the use of commercial fractionating rcolumns,a result which could not be accomplished prior to our invention.

It will be apparent in view of the foregoing 'description that the process of our invention may be modified in numerous ways `without materially departing from the scope thereof. Thus, while the application of our invention has been described principally in connection with the separation of components present in the proportions typical of those found `in the water-soluble chemicals steam obtained `in the hydrocarbon synthesis process, it is to be `strictly understood that our invention is equally applica- -ble to the treatment of mixtures from other sources in which the'commontcomponents of the aforesaid watervsoluble chemicals stream are to be found in similar proportions.

This application is a division of copending application,

U. S. Serial No. 328,181, led December 27, 1952.

`We claim:

1.111 a vprocess for recovering butyraldchyde and fethanol substantially free of methanol from a mixture derivedv by rst subjecting the primary Water stream yprokyduced bythe Fischer Tropsch synthesis yto a first fracentrasse tional distillation step to separate nonacid oxygenated chemicals comprising aldehydes, including butyraldehyde, ketones, including acetone, and alcohols including ethanol, 1-propanol, 1-butanol and methanol in the form of an aqueous overhead containing from about 15 to 20 weight percent water, thereafter subjecting said overhead to further fractional distillation to recover an overhead (l) consisting essentialy of l-propanol, lower boiling components and azeotropic water and leaving as a bottoms stream l-butanol and higher boiling components, overhead (l) containing low molecular weight alcohols not in excess of about 60 weight percent (dry basis), methanol in a concentration of from about 1 to about 10 weight percent based on the weight of the ethanol and acetone in a concentration of from about to about 25 weight percent (dry basis), the steps which comprise subjecting overhead (l) to distillation at a top column temperature ranging from about 73 to about 78.5 C. to produce an overhead (2) consisting essentially of ethanol, methanol, lower boiling components of overhead 1) and not more than about 7 weight percent water; thereafter subjecting overhead (2) to a further distillation step at a top column temperature of from about 54 to about 58 C. to collect an overhead portion containing methanol and lower boiling components, while maintaining said butyraldehyde in the bottoms with said ethanol; and withdrawing a bottoms fraction in which methanol is present in a concentration of less than 0.1 weight percent, based on ethanol present.

2. The process of claim 1 in which said mixture contains from about l to about 5 percent butyraldehyde based on the weight of ethanol and lighter components present therein.

3. The process of claim 1 in which methanol, ethanol, l-propanol and isopropyl alcohol constitute the mixture of water-miscible low molecular weight alcohols ernployed.

4. In a process for recovering ethanol, butyraldehyde and methyl ethyl ketone substantially free of methanol from a mixture derived by first subjecting the primary water stream produced by the Fischer Tropsch synthesis to a first fractional distillation step to separate nonacid oxygenated chemicals comprising aldehydes, including butyraldehyde, ketones, including methyl ethyl ketone, and alcohols, including methanol, ethanol, lpropanol and l-butanol in the form of an aqueous overhead con taining from about 15 to about 20 weight percent water, thereafter subjecting said overhead to further fractional distillation to recover an overhead (1) consisting essentially of l-propanol, lower boiling components and aze0 tropic water and leaving as a bottoms steam l-butanol and higher boiling components; overhead (1) containing low molecular weight water miscible alcohols not in eX- cess ot about 60 weight percent (dry basis) ofthe ethanol and lighter components, methanol in a concentration of about l to about 10 weight percent based on the weight of the ethanol and a mixture of water soluble ketones in a concentration of from about 20 to about 50 percent (dry basis); the steps which comprise subjecting overhead l) to distillation at a column top temperature rang ing from about 73 to about 78.5 C. to produce an overhead (2) consisting essentialy of methanol, ethanol, lower boiling components of overhead (l) and not more than about 7 weight percent water; thereafter, subjecting overhead (2) to a further distillation step at a temperature of from about 54 to about 58 C. to collect an overhead portion containing methanol and lower boiling components while maintaining said butyraldehyde and methyl ethyl ketone in the bottoms with said ethanol; and with drawing a bottoms fraction in which methanol is present in a concentration less than 0.1 weight percent based on the ethanol in said fraction.

5. The process of claim 4 in which said ketone fraction is composed of methyl propyl ketone, methyl ethyl ketone, and acetone, and wherein the latter represents from about to about 75 weight percent of said fraction.

References Cited in the le of this patent UNITED STATES PATENTS 2,382,044 Fisher Aug. 14, 1945 2,454,734 Darlington et al. Nov. 23, 1948 2,588,446 Wilson Mar. 11, 1952 FOREIGN PATENTS 698,617 Great Britain Oct. 21, 1953 

1. IN A PROCESS FOR RECOVERING BUTYRALDEHYDE AND ETHANOL SUBSTANTIAL FREE OF METHANOL FROM A MIXTURE DERIVED BY FIRST SUBJECTING THE PRIMARY WATER STREAM PRODUCED BY THE FISCHER TROPSCH SYNTHESIS TO A FIRST FRACTIONAL DISTILLATION STEP TO SEPARATE NONACID OXYGENATED CHEMICALS COMPRISING ALDEHYDES, INCLUDING BUTYRALDEHYDE, KETONES, INCLUDING ACETONE, AN ALCOHOLS INCLUDING ETHANOL, 1-PROPANOL, 1-BUTANOL AND METHANOL IN THE FORM OF AN AQUEOUS OVERHEAD CONTAINING FROM ABOUT 15 TO 20 WEIGHT PERCENT WATER, THEREAFTER SUBJECTING SAID OVERHEAD TO FURTHER FRACTIONAL DISTILLATION TO RECOVER AND OVERHEAD (1) CONSISTING ESSENTIALLY OF 1-PROPANOL, LOWER BOILING COMPONENTS AND AZEOTROPIC WATER AND LEAVING AS A BOTTOMS TREAM 1-BUTANOL AND HIGHER BOILING COMPONENTS OVERHEAD (1) CONTAINING LOW MOLECULAR WEIGHT ALCOHOLS NOT IN EXCESS OF ABOUT 60 WEIGHT PERCENT (DRY BASIS), METHANOL IN A CONCENTRATION OF FROM ABOUT 1 TO ABOUT 10 